"git fsck" infrastructure has been taught to also check the sanity
of the ref database, in addition to the object database.
* sj/ref-fsck:
fsck: add ref name check for files backend
files-backend: add unified interface for refs scanning
builtin/refs: add verify subcommand
refs: set up ref consistency check infrastructure
fsck: add refs report function
fsck: add a unified interface for reporting fsck messages
fsck: make "fsck_error" callback generic
fsck: rename objects-related fsck error functions
fsck: rename "skiplist" to "skip_oids"
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